The present invention is related to a wind turbine with a yaw system and to a method for the adjustment of the yaw alignment of the wind turbine.
In order to maximise the output of a wind turbine and to reduce the loads on the wind turbine, a rotor of the wind turbine connected to a nacelle is permanently aligned to the wind direction. The wind turbine usually consists of a tower and nacelle that is rotatably mounted on the tower top. The nacelle carries the rotor with at least one rotor blade and a generator, which converts the torque taken up from the wind by the rotor into electrical power.
In wind turbines, it is known to use a motor driven yaw system for the wind direction adjustment of the nacelle. The yaw system is normally disposed between a tower top and the nacelle, wherein the components of the yaw system may be situated in the nacelle as well as in the tower. The yaw system has at least one adjustment drive, which may be equipped with a gearbox, and a yaw bearing, whose inner ring or outer ring has a toothing or is connected to a gear rim. Further, the yaw system is equipped with at least one brake unit, which usually has one or several brake calipers that co-operate with the brake disk(s). The brake calipers each carry brake pads, which sit firmly on the brake disk when the brake caliper is tightened. A pinion connected to the adjustment drive engages with the toothing in order to adjust the yaw alignment of the nacelle, and by this it permits the rotation of the nacelle on the tower top. The adjustment drive is usually fastened on the nacelle, whereas the toothing is made stationary with respect to the tower. After completed yaw adjustment of the nacelle, the nacelle is immobilized by the brake units. These generate the holding torque that is necessary for the nacelle. The holding torque is selected so high that the wind forces acting upon the rotor cannot change the yaw alignment of the nacelle.
In order to avoid oscillation of the nacelle excited by the rotor and by possible turbulences of the wind during the yaw adjustment of the nacelle, the movement of the nacelle is damped during the yaw adjustment. It is known to exert a residual holding torque on the brake disks via the brake units even during the yaw alignment.
From DE 100 23 440 C1, the entire contents of which is incorporated herein by reference, it is known to leave only one or two brake units of the yaw system engaged during the yaw adjustment of the nacelle, in order to ensure the necessary damping of the adjustment process. In this, the adjusting drive is dimensioned such that the nacelle can be adjusted against the applied residual holding torque. Through the residual holding torque of the brakes and the low circumferential velocity in the yaw adjustment, a so-called stick-slip-effect occurs between the brake calipers and the brake disk. The stick-slip-effect occurs in the transition from static friction to sliding friction and vice versa. The oscillations on the tower and the nacelle that occur through the stick-slip-effect lead to mechanical loads. Moreover, the occurrence of the stick-slip-effect is accompanied by the generation of noise. Sometimes the sound emitted by the stick-slip-effect can exceed legally permitted values, so that the wind turbine must be shutdown.
The present invention is based on the objective to provide a wind turbine with a yaw system and a method for the adjustment of the yaw alignment of the nacelle which reliably avoid the occurrence of the stick-slip-effect and of the oscillations and noise emissions accompanied by the same.
The wind turbine according to the present invention has a tower and a nacelle which is rotatably mounted on the tower and whose alignment is adjustable by means of a yaw system. The yaw system has at least one drive, a rotatable yaw bearing, a yaw brake disk and at least two brake units which can exert a holding and/or braking torque on the brake disk. At least one of the brake units has a first brake pad with a first coefficient of friction. According to the present invention, at least one of the other brake units is equipped with a second brake pad which has a second coefficient of friction. According to the present invention, the first and the second brake pad each have different coefficients of friction. By brake units that have different coefficients of friction, it is possible to provide the braking torque that is necessary during the adjustment of the nacelle by one or plural brake units, whose brake pad has a coefficient of friction at which the stick-slip-effect does not appear. Thus, the noise generation during the adjustment of the yaw alignment of the nacelle can be avoided. Even shocks that occur with the stick-slip-effect can be avoided by using two different brake pads in the yaw system.
In a preferred embodiment of the present invention, the coefficient of friction of the first brake pad is smaller than the coefficient of friction of the second brake pad. Due to its static friction between the brake pad and the brake disk, the second brake pad permits to hold the nacelle in a given position. The first brake pad is dimensioned such that a sliding friction appears between brake pad and brake disk. However, the coefficient of friction of the first brake pad is high enough to ensure damping of the adjusting movement.
In a preferred embodiment, the brake units having the first brake pad can be actuated independently from the brake units that have the second brake pad. The independent activation of brake units in the yaw system permits to pressurize the brake units having the first brake pad with such a brake pressure that the movement of the nacelle is damped during the adjustment of the yaw alignment, but the stick-slip-effect can be avoided. In this the brake pressure for avoiding the stick-slip-effect depends on the first coefficient of friction. The brake units with the second brake pad can be selectively pressurised or depressurized.
In a preferred embodiment, the brake units are annularly arranged around a brake disk that is attached on an inner or outer bearing ring of the yaw bearing. Brake units which are actuated hydraulically are preferably used. Alternatively, the brake disk may also be fixed on the tower or the machine frame.
In a preferred embodiment, the brake units having the first brake pad can be pressurized with at least two different brake pressures. In this way, the brake units with the first brake pad can be used in order to enhance the holding torque when immobilizing the nacelle or to apply the braking torque during the adjustment process of the nacelle.
The method of the present invention is used for the yaw adjustment on a wind turbine which has a yaw system with at least one adjusting drive, a rotatable yaw bearing, at least one brake disk and at least two brake units. At least one of the of the at least two brake units is equipped with a first brake pad having a first coefficient of friction, and at least one of the brake units is equipped with a second brake pad having a second coefficient of friction. According to the present invention, the first coefficient of friction is smaller than the second coefficient of friction. In the method of the present invention, a first brake pressure is applied on the at least one brake unit having the first brake pad. Further, the at least one brake unit that is equipped with the second brake pad is released. The method of the present invention further provides to actuate the adjusting drive for adjusting the yaw alignment against the brake pressure that is applied by the at least one brake unit having the first brake pad. In order to hold the nacelle in its predetermined position after the yaw adjustment is completed, subsequently a second brake pressure is applied on the at least one brake unit having the second brake pad. In the method of the present invention, a braking torque adapted to the brake pad is generated by those brake units that have a brake pad with a smaller coefficient of friction, so that the occurrence of a stick-slip-effect is avoided during the adjustment process.
In a preferred embodiment of the method of the present invention, the brake pressure of the brake unit with the second brake pad is subsequently applied also on those brake units that are equipped with the first brake pad. In this way, even these brake units contribute to hold the adjusted yaw alignment of the nacelle.
In a preferred embodiment of the method of the present invention, the first brake pressure which is applied on the brake units having the first brake pad in order to dampen the adjustment movement, is different from the brake pressure which is applied after the completion of the adjustment process in order to immobilize the nacelle, wherein the first brake pressure is preferably smaller than the second brake pressure.
In a preferred embodiment, the first brake pressure which is applied on the brake units having the first brake pad is equal to the brake pressure which is applied to the brake units having the second brake pad in order to immobilize the nacelle. In this case, the first brake pad must have a coefficient of friction which enables damping of the adjustment movement without stick-slip-effect when the brake pressure for generating the holding torque is applied. In case that there is only one value for the brake pressure, the brake units having the first brake pad are permanently pressurized by this brake pressure, whereas the brake pressure on the brake units having the second brake pad is selectively switched to this value or to zero.